Vacuum Web Coating

Delamination problem

In adhesive-laminated 3-ply structures of reverse-printed PET:metPET:LLDPE sealant web, the typical structure failure mode is often peeling of the metallized layer away from its base substrate, even when high adhesion metPET films are used. Some competitive & comparable Japanese and European 3-ply structures do not exhibit this weak peeling or decaling

failure mode, exhibiting outer PET film tear instead (i.e. 'destruct' bonds).

Is this adhesive technology related?

High corona treatment of the metPET?

Can you explain?

ANSWER

Getting high adhesion metallized film can be problematic, particularly as measuring the metal adhesion can be difficult to do well.  Often the only adhesion test done for metallized film is the 'tape' test which is a very poor test which only allows you to eliminate the poorest metallized coatings. 

Part of the problem is the tape test has many variables such as the age and type of tape used, the humidity when the tape was manufactured as well as when it was used, the pressure used to apply the tape and the speed and angle of pull when it is removed, etc.  Thus the test has huge error bars and cannot prove very high adhesion but can only show very poor adhesion.

Corona treatment is used to improve adhesion by increasing the surface energy which improves the wetting if the aluminium as it nucleates and the coating grows.  The corona treatment may not be a reproducible process as it too can be affected by the humidity and so the adhesion can be better in some seasons than others.  The corona treatment also declines with time. The speed of this decline is dependent upon and additives in the film and the temperature of storage.  If there are any additives, such as slip agents used to reduce the coefficient of friction to improve the handling characteristics, these will be contained within the bulk and will migrate to the surfaces as too will any low molecular weight oligomers. These will reduce the surface energy back to the starting level. The higher the temperature and the longer the time the more the benefits of the corona treatment will be lost.   Also as the front surface has a high surface energy immediately after corona treatment it will be energetically beneficial for any low surface energy, low molecular weight material to be transferred from the untreated back surface of the film to the front surface whilst the film is rolled up. Again the longer the film is stored in the roll the greater the opportunity for this material to be transferred again losing the effects of the corona treatment.

If the film also receives a plasma treatment before metallization it may be that the surface becomes over treated.

In general it is beneficial to have a plasma treatment before metallization to correct any reduction of surface energy because of newly migrated or transferred material.  However it is also possible to over-treat the polymer film surface.  It is preferable to optimise the pre-treatment process. If this includes corona as well as plasma treatment then both processes and the length of time between the two processes needs to be optimised as a total item.

If the pre-treatment is gradually increased it will be seen that the surface energy increases up to a maximum and this then plateaus at the high level. If, however, we also plot the adhesion we can see that it initially follows the same path and increases with increasing treatment. However once the maximum is reached instead of remaining high at the plateau level the adhesion immediately starts to decline with any further increase of power or treatment time.

What is happening is that the treatment that causes scission (breaking) of the polymer chains to produce new bonding sites which are often occupied with oxygen which can bind better to the aluminium.  This scission process reaches an optimum in binding sites but any further treatment continues to break chains and this results in ever shorter chain fragments. This finally results in a carbonised layer that is a very weak boundary layer and, although the aluminium may be bonded to it, the adhesion to the polymer bulk is poor because of too many short polymer chain fragments.  The chemical composition stops changing and so the surface energy remains constant at the high level but the adhesion falls away.

Thus for your laminate I would start by reviewing the whole process starting with the polymer film, checking to see if there are any additives included to improve the web handling. I would then go on to check the consistency of the corona treatment, the storage time and conditions of the film following any corona treatment. I would also check if there has been any plasma treatment as well as corona treatment and check to see if the process has been optimised for the film.  (Sometimes the conditions have been set using a different film and it is assumed the same treatment can be applied to other films - and this may not be the case).

I hope this explains what might have been happening and possibly gives some way forward to sorting out the problem.

Question re. lamination problems.

We are doing water-based lamination with metallized BOPP film we also do it with metallized CPP film. However when we do adhesive lamination with BOPP we find problem of corrosion i.e. water attacks metallization layer. We have a doubt that the composition of metallized layer being coated on CPP is different than that of BOPP as they are from two different suppliers. Could you please guide us that what could be there in the metallized layer that is so hydrophilic that it does not allow moisture to evaporate?

Answer.

Cast PP and Biaxially Oriented PP differ where the PP is a mixture of amorphous and crystalline material. The orientation process can result in some alignment within the polymer of the crystalline material. This alignment changes the performance of the polymer is all sorts of ways including tensile performance and also permeation performance.  It is likely that the moisture barrier of the BOPP is better than that of CPP.  This might be a contributing factor.

What you do not mention is what the thickness is of each of the materials and this can also be a factor.  The barrier performance of any material is thickness dependent. Thus, if the CPP and BOPP are of different thickness then this too could affect the performance.

Similarly if the metallization thickness is different in the two samples then this too could result in a differing barrier performance.  Keep in mind that if the materials are metallized by different suppliers there may also be other differences in the metallized layer.  The faster the rate of aluminium deposition the smaller will be the crystal size of the aluminium coating. If the metallization is done at different pressures this too will result in differences within the coating. Higher pressure will result in a less dense coating and lower permeability for the same thickness.  Measuring the coating thickness and also the Optical Density (OD) of each coating will give an indication of how similar the two metallization processes are.  If the thickness were the same but the OD different, or the OD the same but the thickness different it would indicate there are differences in the metallization process. If both thickness and OD are the same, or very similar, it would indicate the metallization processes are also similar.  Another difference could also be in the quality of the coating as described by the number of pinholes per unit area.  The higher the level of pinholes the greater the permeability of the coating as well as the greater the number of starting points for any corrosion.  The surface roughness of the substrate can also affect the nucleation and growth of the aluminium and hence the barrier performance. The higher the surface roughness the worse the barrier performance is likely to be.  Other factors that could affect the corrosion of the metallized film are the temperature and humidity conditions seen by the roll of material.  If the metallized film is re-wound hot, as in greater than 30 deg C, in the metallizer it is likely that the aluminium coating will grow a thicker oxide more rapidly than if the temperature at the re-wind is closer to ambient, generally less than 30 deg C.  After metallization if the rolls are stored in a high humidity atmosphere they will hydrate the aluminised film and lead to more rapid oxidation than if stored in a low humidity atmosphere. Adhesion is another possible factor that could have an effect. It would be expected that the system with the lower adhesion to suffer from higher permeation and more likely to have a greater propensity to corrosion.   I would normally expect the metal to CPP adhesion to be lower than that of metal to BOPP.   

I would suggest that measuring the barrier performance, OD, metal thickness and adhesion of each of the metallized materials before lamination would be a useful comparison. 

I would expect that the barrier performance of the BOPP would be higher than that of the CPP thus making the removal of the water from the adhesive slower than for the CPP.

I hope that this gives you some points that you find useful in helping solve this problem.

Problem with low bond strength

We are metallized film with plasma by applied a 3.0 to 4 kW power. We are

using 1200 sccm of oxygen & 400 sccm of argon combination.

With above combination we are facing problem of low metal bond strength in

metallization done on corona treated side film.

If we try to increased the power of plasma more then 5 or 6 kW, plasma get

tripped & unable to start for next 15-20 min.

I would like to understand that at what power range will give best metal

adhesion?

We are doing metallization on 12 mic with 2.65 optical density. Maximum

width of metallization is 2450mm.

Pl reply with possible cause of low metal bonds, cleaning frequency of

plasma, gas combination & rate of gas, power etc.

ANSWER
pre-treatments are one of the most frustrating parts of the process because it can be so variable, often varying with changes in the weather.

Corona treatment is aimed at increasing the surface energy of the polymer film. So too is the in-vacuum plasma treatment.

Using either or both of these processes there are three possible outcomes.

1.    The treatment has little or no effect and the adhesion is little better than with no treatment.

2.    The treatment delivers a higher adhesion.

3.    The treatment has some effect but sometimes the adhesion is worse than using no treatment.

The pre-treatments are used to do a variety of different things to the.  It can be used to help remove loosely bound material from the surface, also low molecular weight material that has migrated to the polymer surface can also be removed or carbonised or cross-linked into the polymer surface, also the polymer surface can be chemically modified to enhance both the wetting and adhesion.

What you are trying to achieve is a maximum value for the surface energy. However a simple measure of the surface energy can be misleading for the following reason.  There will be a surface energy associated with the untreated polymer film. Now as the pre-treatment is done the surface energy should be higher than the untreated film.  As you have a roll of material optimising this process should be done quite easily by winding material through and progressively increasing the power to the corona treater or in-vacuum plasma treater.  It is then possible to measure the surface energy at each power level.  What you should see is a progressive increase in the surface energy with power to a point where the surface energy levels off at some maximum value.

       It is this maximum value that is misleading.  If you also plot the adhesion of a coating or metallization it will follow a similar curve except that where the surface energy levels off and continues with higher powers (or longer treatment time) at an almost constant high value, the adhesion instead of levelling off it reaches a peak and then almost immediately falls off with increasing power or treatment time.

    The reason for this fall off of the adhesion is that the surface of the polymer has been overtreated. The treatment is often a balance between chain scission that generates new bonding sites and aids adhesion and the scission that creates new short chain molecules that are short enough to be weakly bonded into the bulk polymer and hence form a new weak boundary layer. If the power is further increased these short chain molecules tend towards carbon molecules, thus the weak interface is made up of an excess of carbon even if high oxygen content plasma is used.

The use of corona and in-vacuum plasma can make it easier to overtreat the surface in some circumstances.

Often there is some time between the corona treatment and metallizing process. During this time it is common for low molecular weight materials to migrate back to the surface and re-contaminate the surface and so the in-vacuum plasma treatment is essential.  However if the same material is both corona treated and vacuum plasma treated in the same day it may be that the surface is overtreated.   Thus it becomes important not only to know what the treatments are but the time between the treatments can become equally important.

There are other variables that also need to be monitored and understood. The temperature and humidity can affect both the process and the polymer.  The corona treatment at the same power can produce different results when the humidity is high compared to when the humidity is low.

Winding the polymer in high humidity will trap more water in the roll than winding in low humidity. This is released in vacuum and will also vary the gas content of the plasma treatment process.

I am slightly surprised that the oxygen flow is so much higher than the argon.  Mostly I see argon being the larger flow and oxygen being somewhere in the 10% - 20% range.  The argon provides the heavy ions for doing the chain scission and the oxygen provides the bonding to the carbon by-products making then volatile and capable of being pumped away and also the oxygen will bond onto the polymer where fresh chain ends have been created which can also improve the bonding to the aluminium.

So for your problem I would start by checking the optimisation of the process. Making sure that the surface energy has been maximised but also making sure that the polymer has not been overtreated and that the surface has not been carbonised.

I would also look at the variability in the process. What are the variations in the humidity and differences in time between corona treatment and metallizing?  Look at the optimisation for a constant corona power but with highs and lows of humidity and time between processes. This should give you some idea how much the process can vary and what might be done with the vacuum plasma treatment to compensate for the variations.

I hope this gives you something to work with.

Moisture and aluminium performance

Can moisture affect the performance of aluminium metallized films?

Answer.

Yes moisture can affect the performance of aluminium metallized films. This starts with the film before metallization. Most polymers contain moisture as well as the water in the air that is trapped between layers as it is wound into a roll.  This is all carried into the vacuum system. Even when the system is pumped out to a low base pressure there will still be enough oxygen and water in the system that a monolayer of oxygen can form on a surface in less than 1 second.  Thus all aluminium metallized films have a proportion of oxide in them, usually of the order 1% - 2%.  If the vacuum system has a leak this can be somewhat worse.

The aluminium is a metal that forms an oxide on the surface that acts as a good barrier layer and prevents further rapid oxidation.

The adhesion of the aluminium to the polymer web is dependent on a number of things. This can include the polymer quality, contamination, the storage conditions (humidity and temperature), any surface treatment, the type of treatment, the age of any treatment as well as the process conditions such as deposition pressure.  When laminating another layer to the surface it will depend on the relative adhesion strengths and the residual stress following the lamination. If the residual stress is large the adhesion of the aluminium needs to be higher than if the residual stress is low.

The speed of oxidation and amount of oxidation depends on the thickness of the aluminium layer. If the aluminium is very thin and the adhesion is poor the there will be very little aluminium left once the surface has been oxidised. If the aluminium is thicker then even after the surface oxidation there will be sufficient metal left to give a long lifetime for the rest of the aluminium.

Problem re. Ink adhesion to metallized film

In fact we first laminate the metal side of METPET in to 1.5 mil Polyethylene film using solventless adhesive of Liofol 7980/7275.
Once it is cured, in second process we surface print on PET side of the structure.
Following is the final structure from outside to inside:
                                        OPV (OVER PRINT VARNISH)
                                        INK
                                        PET SIDE (OF METPET)
                                        METAL SIDE (OF METPET)
                                        ADHESIVE (SOLVENTLESS)
                                        POLY SEALANT FILM
DEFECT:
        The ink adhesion is good but with slight twist of the film the ink is released from the PET showing Missout and a kind of pinholing showing metal.
        This happens mostly in shipping and in handling the package at customer plant.
       
        If we try to separate the layers, the metal is released from the PET but the do not delaminate with Poly sealant layer.

       How to have 100% ink adhesion on PET of METPET?

Forgot to mention one important information, sorry.
The metal on METPET is on the corona treated side.

Answer

It sounds as if the possible problem is the same for the ink as for the metal.

Corona treatment is helpful to a certain extent but is a variable treatment.  When PET is manufactured there is always some residual unpolymerised material left in the polymer. This oligomer is low molecular weight and is easily able to migrate through the polymer and it will appear on the surface.  The amount on the surface will depend on the age and storage conditions of the polymer roll.

If untreated and left on the surface both the ink and the metal will bond to this oligomer, which forms a weak boundary layer, and will have a poor bond strength.

Corona treatment will improve this bond but has limitations.  The corona treatment needs to be optimised and also it needs to be done immediately before coatings are applied.  The corona treatment varies with humidity and so what might be optimised on a dry day with low humidity may not be optimised for a very humid day.  The treatment is also not a permanent treatment.  The oligomer will still be present within the bulk of the polymer and with time and/or temperature it will migrate back out to recontaminate the surface. 

Thus if the corona treatment is done before the metallization on both sides of the PET the metal adhesion may be at one level but as the film will have been heated during the metallization process the back surface treatment may already be recontaminated by the time the film leaves the metallizer. Similarly if the lamination is done using heat there will be a further migration of material to the PET surface.

The indication you have given is that the ink is printed onto a surface that has not been corona treated. Thus the ink adhesion could be improved using a corona treatment before the printing.

I personally would not rely on the corona treatment for the metallization but would consider using a plasma treatment within the metallizer, if at all possible, so that the treatment is immediately before the metallization and there is less chance of re-contamination. I would also use an oxygen containing plasma to make sure that I removed as much organic contamination as possible. An argon plasma has no mechanism for removing organic material. It just knocks material off the surface which can re-deposit on the surface. Oxygen will convert organics into volatile species that can be pumped away by the vacuum pumps. Also the oxygen will form bonds on the surface that will also form bonds with the aluminium and increase the bond strength.

If you cannot carry out a plasma treatment but only have corona treatment as an option I would make sure the process has been optimised and would check that this has been optimised at different humidity levels.  I would also make sure that the time the roll is stored between corona treatment and metallization is minimised again to minimise the time for recontamination. 

Re-contamination occurs by two processes one is the migration of material from the bulk as mentioned above and the other is by transfer of material from one surface to another. Bear in mind that, as the back surface has not been treated, the low molecular weight, and often low surface energy material on the back surface will be brought in contact with the high surface energy front surface as the film is re-wound. As nature tries to equilibrate energies it is preferable for some of this low energy material to transfer from the back surface to the front surface.

This is the reason for minimising the time the film is standing between corona treatment and metallizing.

Hopefully this explains a possible source of variable adhesion and some options to correct the problem.

Barrier performance and storage time

Is the barrier performance of properly stored metallized film affected by ageing ?
When should the barrier properties of metallized films be checked ?
Immediately after metallization or after ageing (time)?

Answer.

The barrier performance can change with time. Particularly for aluminium metallized film. The aluminium will gradually be oxidizing and so the OD might fall slightly with time and so to the barrier might change with time.  Also the adhesion might change with time dependign on the adhesion, any pre-treatment and the contant of the polymer. If the polymer has residual oligomers or slip agents included and the adhesion is not optimised these low molecular weight materials can exude to the interface and reduce the adhesion. If you have reduced adhesion there is an increased possibility of loss of aluminium due to movement of debris on the surface or pick-off to the back surface either of which can cause the barrier performance to degrade.

The safest time to measure the barrier performance is immediately prior to shipping as then you will have the best idea of what your customer is going to receive. Your values may still not match their values because they too may store the material for another period of time or they may rewind it before measuring and rewinding gives another opportunity for debris ot pick-off to produce more pinholes and hence further degrade the barrier measurement.

I hope this helps.

Problem of delamination

Recently we observe delamination in liquor sachets.  We use high performance adhesive for this purpose. When the liquor is filled in the laminate there is no delamination.  But, even if one sachet leaked it delaminates all the other sachets. What could be the reason for it & how we can avoid it?

Answer

There are a couple of things that might be worth thinking about. One is the adhesion is only as good as the weakest part of the lamination interface. You have two (probably different materials) that have a surface that can affect the adhesion as well as the adhesive. Although you have chosen a high performance adhesive it can still be affected by a poor surface on either of the two surfaces it is trying to laminate together. The surfaces may be contaminated or have a low surface energy. This can mean that the adhesive does not wet the surface well or does not stick to the surface well. It may still stick better than a lower performance adhesive but still not as good as it might.

Contamination in this case would be things such as the additives used to control the coefficient of friction as used in materials such as OPP, which lowers not only the coefficient of friction but also the surface energy. If the surface energy is low the adhesive may not wet the surface fully and so there will be gaps between the adhesive and film where liquid could migrate easily. Also the area in direct contact is less and so the adhesion strength falls.

The chances are that the inner material is well matched to the liquor you are enclosing however the outer material may have a lower performance. One possible way that additional stress can be put on the laminate is that the liquor is taken up by one of the materials and it swells putting an additional stress on the lamination adhesive. If the adhesion is not good enough this stress can then cause delamination. In the same way if the outer material allows the liquor to migrate through to the lamination adhesive and the adhesive is affected by the liquor then the adhesive may swell or lose some of its adhesive properties and again the swelling can put too much load on the joint allowing delamination.

I would check out how the liquor interacts with each material, checking to see if it swells the film or adhesive and how if affects the adhesive tack strength.   I do not know what the liquor is but does it include a solvent and check to see if the materials are compatible to that solvent.

I hope this gives you a couple of things to look into.

Film storage conditions

Can you provide us any information on the shelf life and proper storage conditions of vacuum metallized films (in rolls)? And what will deteriorate in the laminate over time, what are the factors that can cause the deterioration of the vacuum metallization?

Answer

To define proper storage conditions can be difficult. Different areas in the World have different problems and hence requirements. However there are some basic principles that can be applied. It is preferable to have the temperature and humidity regulated. The reason for this is that many polymers will absorb and desorb moisture and all will expand and contract with temperature. Any relative movement can induce surface damage such as microscratches as the layers of film move very slightly against each other. This can be due to any protruding filler or because of the residual debris on the surface that may also lead to pinholes.

Large temperature changes can also lead to film handling problems as if the roll is moved from one temperature to an area with a very different temperature part of the roll may have expanded but not the rest and if moved the roll will be prone to telescoping. If the temperature becomes very low the cores can be crushed particularly after metallizing where the roll will already be hard because the rewind in vacuum does not provide any interleaving air to soften the roll.

High temperature can also be a contributing cause of delamination problems in some materials.  Many films contain oligomers and additives such as slip agents. These low molecular weight materials will migrate from the bulk of the polymer to the surface. If the metal is not well adhered this low molecular weight material will appear at the interface and the adhesion will fall. This may well also occur with laminates but it does depend on the material adhesion and the wetting characteristics of the any coating.  Thus the aim is always for optimised adhesion where possible so that the coating wets the surface well and the adhesion is high. This will lead to a more stable product long term as there are fewer points where the low molecular weight materials can exude into.

In controlling humidity this can lead to other problems. It would be nice to have very dry conditions as this would keep the moisture level in the polymers low and this could help the vacuum metallizing process and possibly reduce the speed of post deposition aluminium oxidation.  However if there is winding processes in the same environment there will be significant electrostatic charging and the concomitant winding problems and possible film damage from uncontrolled discharges. Thus the humidity needs to be sufficient to keep these electrostatic problems under control.

Hence in some parts of the World the controlled atmosphere usually means cooling and drying the atmosphere whereas in others it is preferable to heat and moisten the atmosphere.

I would monitor your seasonal changes in temperature and humidity and pick a suitable mid-point (allowing for a workable humidity level) and then look to stabilise the temperature and humidity around that.

I am sorry it is not a definitive answer but hopefully it is one that you can work with reasonably easily.

Problem with bond strength.

Is VMCPP with high optical density (2.4 - 2.6) will have lower bonding strength compared to VMCPP with lower density (1.8 - 2.0)?

If yes, please explain why and how to overcome this problem?

Answer.

It is most likely that the adhesion is the same for both coatings but the stresses on the thicker coating means that it appears to fail more readily.  Essentially you have two dissimilar materials and they have very different coefficients of thermal expansion/contraction so that there is always likely to be some stress between the coating and the substrate. The thinner the coating the less noticeable this is likely to be. As the coating thickness increases there is likely to be an increase in the tendency for the material to curl and in keeping the web under tension to keep the web flat will increase the load at the substrate/coating interface. Also as the metal does not stretch like the polymer the tension will also add a load to the interface. If the adhesion has any weakness the coating will fail at this point.

Many polymers have a surface that is slightly different to the bulk polymer. It is possible that the failure is occurring within the polymer and not at the metal/polymer interface. This surface layer may be due to material exuding out of the surface such as slip agents or other additives or it could be due to unpolymerised residues.  These are a source of weakness and it is these that pretreatments are used to try to minimise.  So it is worth checking on the pre-treatment, find out what has been done, if any, and then as some of these treatments can be temporary check to see how old the treatment is and ideally make sure there is a plasma treatment in the metallizer immediately before the metal deposition and make sure this final treatment has been optimised.  It is possible to over treat the surface and this can degrade the adhesion too hence it is worth checking the treatment is optimised.

Which side has been metallized?

Request: I have rolls of holographic film delivered to my plant. How can I
tell which side is the metal side out?

Answer.

Generally using a knife-edge to gently scratch the surface will tell you which side is metallized. On the non-metallized side the scratching will produce white dust only and when held up to the light the metallized optical density will be unchanged. When scratching the metallized side the dust may appear darker and, more importantly, when the film is held up to the light will shine through much brighter where the metal has been removed and the optical density has been reduced. It is important to scratch with a series of gentle strokes. You are only trying to remove a few nanometers of the surface and not peel away microns of the surface. I have see a heavy-handed person dig through from the non-metallized side determined to scratch through to the metal.  If a lacquer has been applied it may take additional scratching to remove the lacquer before you get to the metal but this is still going to be relatively thin and the difference should still be obvious.

If the adhesion is not good then using the sellotape test, where sellotape is stuck on the surface and pulled off.  If the metal adhesion is poor then on the metallized side some will be removed and again holding the film up to the light will show this up.
This only works for poor adhesion samples and where this does not work the scratching test has to be resorted to.